Checking If a Host is Alive

Checking if a host is alive is a basic test that should be performed for all remote machines. Nagios offers a command that is commonly used for checking if a host is alive and plugged into the network. The syntax of the plugin is as follows:

check_ping -H <host_address> -w <wrta>,<wpl>% -c <crta>,<cpl>%
           [-p packets] [-t timeout] [-4|-6]

This command accepts the standard options described above, as well as the following nonstandard options:

RTA means Round Trip Average, and is the average time taken in milliseconds for the package to return. PKTLOSS is Packet Loss, which is the maximum percentage of packages that can be lost during communication. For example, a value of 100, 20% means that a ping must return within 0.1 seconds on average, and at least 4 out of 5 packages have to come back.

A sample command definition for checking if a host is alive is:

  define command
  {
    command_name  check-host-alive
    command_line  $USER1$/check_ping -H $HOSTADDRESS$ -w 3000.0,80%
                  -c 5000.0,100% -p 5
  }

Testing Connectivity over TCP and UDP

In many cases, Nagios is used to monitor services that work over the network. For checking if a service is working properly, it is necessary to make sure that a certain TCP or UDP port is accessible over the network. For example, Microsoft SQL Server listens on TCP port 1433. In many cases, it is enough to simply run generic plugins that check whether a service is available on a specified TCP or UDP port. However, it is recommended that you run specialized plugins for various services such as web or email servers, as these commands also try basic communication with the server and/or measure response time.

Internally, as this command is also handling many other checks, the syntax is almost the same. It is designed so that, it behaves slightly differently based on the name it is called with. Many other plugins are symbolic links to check_tcp. The check_tcp plugin is mainly intended to test services that do not have a corresponding Nagios check command. The second command, check_udp is also a symbolic link to check_tcp and differs only by communicating over UDP instead of TCP. Its syntax is as follows:

check_tcp|check_udp -H host -p port [-w <warning >] [-c <critical >]
          [-s <send string>] [-e <expect string>] [-q <quit string>]
          [-A] [-m <maximum bytes>] [-d <delay>] [-t <timeout>]
          [-r <refuse state>] [-M <mismatch state>] [-v] [-4|-6] 
          [-j] [-D <days to cert expiry>] [-S] [-E]

These commands accept several nonstandard options as follows:

An example, to verify whether VMware server 1.x is listening for connections, is as follows:

  define command
  {
    command_name  check_vmware
    command_line  $USER1$/check_tcp -H $HOSTADDRESS$ -p 902
                  –e "220 VMware"
  }

For UDP, the following is an example command definition to verify if the OpenVPN server is listening on UDP port 1142:

  define command
  {
    command_name  check_openvpn
    command_line  $USER1$/check_udp -H $HOSTADDRESS$ -p 1142
  }